Chemistry Reference
In-Depth Information
open apparatus
was chosen. If one takes a
closed apparatus
, weighs all
educts first and later all reaction products, one will see that masses are not
changing, but are conserved. Lomonossov and Lavoisier were the first to
have found this law of conservation of mass in the eighteenth century.
Dalton explained it in 1808 by combining the idea of elements with the
idea of atoms, and stating that no atom can be created nor destroyed, instead
atoms are rearranged in every chemical reaction.
Material: Digital scales, test tubes, balloons, burner; iron wool, matches.
Procedure: (a) A test tube is filled with iron wool, closed with a balloon and
weighed on the balance. After heating the test tube with the burner and
observing the iron wool, the test tube is weighed again once it cooled down
to room temperature.
(b) A test tube is filled with five to eight matches, closed with a balloon and
weighed on the balance. After heating the test tube close to the match heads
with the burner and observing the matches, the test tube is weighed again
after cooling it down to room temperature.
Observation: The balloon gets bigger and bigger; the iron wool glows red
and black iron oxide is formed. The match heads are burning with a bright
flame. In both cases the mass is the same before and after the reaction.
E1.9.
Conservation of Matter by Carbon Combustion
Problem: The experiments with iron wool and matches show that combus-
tion products like solid iron oxide or the wooden remnants of the matches
remain. In the case of carbon or charcoal nothing will remain and young
students may think: “if the carbon is totally gone there is nothing to be
weighed, the law of conservation of mass cannot be valid.” So it seems very
important that the gaseous combustion product carbon dioxide must be
shown and weighed to convince students that the law is valid even in this
case. The next experiment (E1.10) will also show that gases have specific
densities, that the density of carbon dioxide is higher than that of air.
Material: Digital scales, 2 L round bottom flask, balloon, burner, gas syringe
with 20 cm glass tube, test tube; small pieces of charcoal, oxygen, lime water.
Procedure: The flask is filled with oxygen and 3-5 small pieces of charcoal,
then closed with a balloon and weighed exactly. With the burner the pieces
of carbon are heated from outside until they ignite, then the flask is turned
around and around to show the glowing carbon pieces. After complete
combustion the flask has to be cooled down and weighed again. The flask
can be opened, with the syringe 100 mL of the gas is taken out of the flask
and bubbled through 1-2 mL of lime water in the test tube.
Observation: The balloon gets bigger and bigger, and the carbon pieces
glow very bright and disappear totally. The gas produces a milky precipita-
tion in lime water: carbon dioxide.
E1.10.
Density of Air and Carbon Dioxide
Problem: Students are well aware of the existence of air around them, but
not aware of air as a space-filling substance with special properties and
measurable density. This density should be measured and compared to the
density of carbon dioxide. The density of air can also be discussed in the
context of air pressure (see E1.1).
